Ah, that’s probably it then. I was playing the game directly from my Downloads folder and the system never prompted me to grant it write access.
Sure!
BTW, in the future it would probably be better to write save data into a directory in ~/Library/Application Support/ or ~/Library/Caches or the like (e.g. ~/Library/Application Support/Caterpillar Comittee/) since that’s what macOS expects for non-document save data (and they expect documents to be written out through the system file dialog). Apple is getting really strict about their security policy these days.